The main thing that is missing in your diagram is a sense of how far through the billing period the chart represents. I'm not convinced that a number of days remaining is a good way of communicating that either.
The problem for your users, presumably, is whether or not they're on track to go over their data allowance and get shaped, or if they can keep going at their current pace.
You could consider either showing another chart beneath it at the same scale representing progress through the billing period (e.g. If its monthly billing and I'm 18 days through the month but have used 100 GB, the period chart will be behind the usage chart, indicating that the user is on pace to be shaped and thus should change their behaviour), or a two dimensional chart with time on one axis and allowance on the other.
This second option would effectively look like a burndown chart, with the data remaining shown on the left and the days on the bottom, and as per a burndown chart you could fairly easily indicate how much usage per day could be accommodated under the user's cap, and how many days of shaped speeds the user will have to endure based on their current usage trajectory.
As far as communicating the effect of exceeding the cap, I don't believe that is useful to present on the chart itself as much as an annotation or note beside it indicating that any overage will not incur a fee but will result in a slower connection.
To give you an idea of what I mean, here's a chart I threw together in Excel based on the data in your example:
I'm sure you can increase the information density given the real data and your actual billing periods, and the visual design leaves a bit to be desired, but hopefully you get the point.
As mentioned above, I think it you should probably think hard about how much value the on-chart annotations have (as opposed to simply listing that information off to the side of the chart and allowing the chart to speak more or less for itself).
That option has the side benefit of being accessible to people who can't load the chart for some reason or another (the three annotations listed in plain text on the page effectively describe the most important information in the chart in a non-visual way).